RF voltage and current (V-I) sensors and measurement methods

ABSTRACT

A radio frequency (RF) system includes a RF power source configured to power a load with an RF signal; an RF pipe including an inner conductor and an outer conductor connected to ground coupling the RF power source to the load; and a current sensor aligned to a central axis of the RF pipe carrying the RF signal. A sensor casing is disposed around the RF pipe, where the sensor casing includes a conductive material connected to the outer conductor of the RF pipe. A gallery is disposed within the sensor casing and outside the outer conductor of the RF pipe, where the current sensor is disposed in the gallery. A slit in the outer conductor of the RF pipe exposes the current sensor to a magnetic field due to the current of the RF signal in the inner conductor of the RF pipe.

This application is related to co-pending U.S. Non-ProvisionalApplication Ser. No. 16/913,545, filed on Jun. 26, 2020, and U.S.Non-Provisional Application Ser. No. 16/913,548, filed on Jun. 26, 2020,which applications are hereby incorporated herein by reference.

TECHNICAL FIELD

The present invention relates generally to plasma processing systems andmethods, and, in particular embodiments, relates to radio frequency (RF)voltage and current sensors and measurement methods.

BACKGROUND

Generally, advancements in semiconductor integrated circuits (IC's) aredriven by a demand for higher functionality at reduced cost. Higherfunctionality at lower cost is provided primarily by increasingcomponent packing density through miniaturization. An IC is a network ofelectronic components (e.g., transistor, resistor, and capacitor)interconnected by a multilevel system of conductive lines, contacts, andvias. Elements of the network are integrated together by sequentiallydepositing and patterning layers of dielectric, conductive, andsemiconductor materials over a semiconductor substrate using afabrication flow comprising process steps such as chemical vapordeposition (CVD), photolithography, and etch. The packing density ofcircuit elements have been increased by periodically reducing minimumfeature sizes with innovations such as immersion lithography andmultiple patterning. Further miniaturization is achieved by reducing thedevice footprint with three-dimensional (3D) device structures (e.g.,FinFET and stacked capacitor memory cell).

Plasma processes such as reactive ion etching (RIE), plasma-enhanced CVD(PECVD), plasma-enhanced atomic layer etch and deposition (PEALE andPEALD), and cyclic plasma process (e.g., cycles of alternatingdeposition and etch) are routinely used in the deposition and patterningsteps used in semiconductor IC fabrication. The challenge of providingmanufacturable plasma technology for advanced IC designs, however, hasintensified with the advent of feature sizes scaled down to a fewnanometers with structural features controlled at atomic scaledimensions. A manufacturable plasma process is expected to providestructures with precise dimensions (e.g., linewidths, etch depth, andfilm thicknesses) along with precisely controlled features for bothplasma etch (e.g., sidewall angle, anisotropy, and selectivity toetch-stop layers) and plasma deposition (e.g., conformality,aspect-ratio selectivity, and area selectivity for bottom-uppatterning), and uniformity across a wide (e.g., 300 mm) wafer. In manyof the plasma processes used in IC manufacturing, the plasma issustained by RF power. Since the plasma properties are influenced by theRF power delivered to the processing chamber, precise control of plasmaprocesses may need innovative metrology of RF signals that areunobtrusive and accurate.

SUMMARY

In accordance with an embodiment of the present invention, a radiofrequency (RF) system includes a radio frequency (RF) power sourceconfigured to power a load with an RF signal; an RF pipe including aninner conductor and an outer conductor connected to ground coupling theRF power source to the load; and a current sensor aligned to a centralaxis of the RF pipe carrying the RF signal. The current sensor isconfigured to monitor the current of the RF signal, and includes aconductive half-loop disposed proximate the RF pipe, where theconductive half-loop includes a first end and an opposite second end.The current sensor is configured to output an output signal between thefirst end and the second end. A sensor casing is disposed around the RFpipe, where the sensor casing includes a conductive material connectedto the outer conductor of the RF pipe. A gallery is disposed within thesensor casing and outside the outer conductor of the RF pipe, where thecurrent sensor is disposed in the gallery. A slit in the outer conductorof the RF pipe exposes the current sensor to a magnetic field due to thecurrent of the RF signal in the inner conductor of the RF pipe.

In accordance with an embodiment of the present invention, a radiofrequency (RF) system includes a radio frequency (RF) power sourceconfigured to power a load with an RF signal; an RF pipe including aninner conductor and an outer conductor connected to a referencepotential node coupling the RF power source to the load; and a firstvoltage sensor disposed axisymmetrically around an axis of the RF pipecarrying the RF signal, where the first voltage sensor is configured tomonitor the voltage of the RF signal.

In accordance with an embodiment of the present invention, a method ofmeasuring a radio frequency (RF) signal includes having a current sensoraligned to an axis of an RF pipe carrying an RF signal, where thecurrent sensor is disposed in a gallery that is disposed within a sensorcasing and outside an outer conductor of the RF pipe. The sensor casingis disposed around the RF pipe. The current sensor includes a conductivehalf-loop, where the conductive half-loop includes a first end and anopposite second end. The method includes determining a current of the RFsignal based on measuring an output signal between the first end and thesecond end.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention, and theadvantages thereof, reference is now made to the following descriptionstaken in conjunction with the accompanying drawings, in which:

FIG. 1A illustrates a block diagram of a generic plasma processingsystem for semiconductor IC fabrication;

FIG. 1B illustrates a cross-sectional view of a V-I sensor for an RFpipe, in accordance with an embodiment;

FIG. 1C illustrates a top-sectional view of a current sensor of a V-Isensor for an RF pipe, in accordance with an embodiment;

FIG. 2A illustrates a perspective view of a V-I sensor for an RF pipe,in accordance with an embodiment;

FIG. 2B illustrates a cutaway diagram of the V-I sensor illustrated inFIG. 2A;

FIG. 3 illustrates a cutaway diagram of a V-I sensor for an RF pipe, inaccordance with some embodiment;

FIG. 4 illustrates a cutaway diagram of a V-I sensor for an RF pipe, inaccordance with some embodiment;

FIG. 5 illustrates a cutaway diagram of a V-I sensor for an RF pipe, inaccordance with some embodiment;

FIG. 6A illustrates a perspective view of a V-I sensor for an RF pipe,in accordance with some embodiment;

FIG. 6B illustrates a cutaway diagram of the V-I sensor illustrated inFIG. 6A;

FIG. 6C illustrates a cross-sectional view of the V-I sensor illustratedin FIG. 6A;

FIG. 6D illustrates a perspective view of a current sensor element ofthe V-I sensor illustrated in FIG. 6A, in accordance with oneembodiment;

FIG. 7A illustrates a perspective view of a current sensor assembly foran RF pipe, in accordance with some embodiment;

FIG. 7B illustrates an exploded view of the current sensor assemblyillustrated in FIG. 7A;

FIG. 7C illustrates an exploded view of a cutaway diagram of the currentsensor assembly of FIG. 7A, along with an RF conductor for an RF pipe;

FIG. 7D illustrates a cutaway diagram of the current sensor assemblywith an RF conductor for an RF pipe illustrated in FIG. 7C, inaccordance with some embodiment; and

FIG. 7E illustrates a planar view of a bottom portion of the currentsensor assembly with an RF conductor, illustrated in FIG. 7C.

The last two digits of all the three digit reference numerals in FIG. 1Athrough FIG. 7E always represent similar components.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

The making and using of embodiments of this disclosure are discussed indetail below. It should be appreciated, however, that the conceptsdisclosed herein can be embodied in a wide variety of specific contexts,and that the specific embodiments discussed herein are merelyillustrative and do not serve to limit the scope of the claims.

This disclosure describes sensor designs and methods used for accuratelymeasuring voltage (V), current (I), and the phase angle (Φ) betweenvoltage and current of radio frequency (RF) electrical signals. Theembodiments of the voltage-current (V-I) sensors described herein havebeen applied to probing the electric and magnetic fields of RFelectromagnetic waves along coaxial transmission lines, referred to asRF pipes.

Plasma processes in semiconductor fabrication (e.g., plasma etch anddeposition processes) often use RF power to sustain the plasma. As knownto persons skilled in the art, the RF signal sustaining the plasma inthe plasma chamber influences the plasma properties. The plasmaproperties (e.g., electron density, plasma sheath thickness, ratio ofion to radical flux, and others), in turn influence the etching and/ordeposition characteristics of the plasma process.

In various embodiments, this application describes RF V-I sensors formeasuring the current and voltage of the RF signal through an RF pipe.The term RF pipe refers here to a coaxial transmission line whichcarries RF power from one portion of a plasma reactor (referred to as aplasma processing system) to another. The waveforms, I and V,transmitted through an RF pipe are functions of position (x) and time(t), I(x, t) and V(x, t). When a single frequency component, ƒ, ispresent, the current and voltage are described by sinusoidal waveformswritten compactly as Re(I(x)e^(j) ^(ω) ^(t)) and Re(V(x)e^(j() ^(ω)^(t+) ^(Φ) ⁾), where ω=2πƒ, j²=−1, and Re is the real part of thecomplex functions. As mentioned above, I and V each has a magnitude andeach is separated from the other by a phase angle, Φ. In general, thewaveforms, I and V, may include multiple frequency components. Thevoltage V(x, t) refers to the potential of the inner conductor (or core)of the RF pipe (or coaxial transmission line) relative to the groundedouter conductor (or shield), where ground denotes the referencepotential of the RF system.

The RF V-I sensors, as described in various embodiments, may usegeometrical symmetry and differential measurement methods to provide Vand I with high precision at the position of the sensor. Several V-Isensors may be used at various locations along the RF pipes to probe theRF signals there. Since V and I are functions of position, x, each V-Isensor may be positioned as close as possible to the respective desiredmeasurement location. For example, if it is desirable to monitor andcontrol the plasma process using accurate measurements of the voltageand current of the RF signal provided to the plasma chamber then the V-Isensor used for that purpose may be positioned close to where the RFsignal enters the plasma chamber. The various embodiments describedherein enhance the measurement accuracy and sensitivity of V-I sensorswithout increasing cost, thereby providing plasma processing systemsenhanced ability to provide plasma processes with better repeatabilityand tighter process control for the same cost. Additionally, the RFprobes are designed to be unobtrusive to allow ease in retrofittingexisting plasma processing equipment with the improved V-I sensorswithout time-consuming and expensive re-development of establishedrecipes for plasma processes in a production flow.

In this disclosure, the use of V-I sensors in plasma processing systemsis explained first, with reference to a block diagram illustrated inFIG. 1A. Next, the basic structure and operating principle of a V-Isensor in a plasma processing system is described with reference to theschematic illustrated in FIG. 1B in accordance with an embodiment. Someof the innovative aspects of the V-I sensor designs (designs similar tothe basic structure in the schematic in FIG. 1B) are then explained withreference to example embodiments of V-I sensors illustrated in FIGS. 2A,2B, 3, 4, and 5 .

As described in detail below with reference to FIG. 1B, the voltagesensors in the embodiments in FIGS. 2A, 2B, 3, 4, and 5 have axialsymmetry, being designed as conductive rings placed inside the RF pipelooping around the central longitudinal axis that runs parallel to thedirection of current flow. The advantages provided by the axisymmetricdesign have been explained in the discussion below with reference toFIG. 2B.

The respective current sensors in the example embodiments are located ina sleeve or gallery around the circumference outside of the RF pipes.The gallery is the cavity inside a sensor casing. The sensor casing hasconductive walls that cover the gallery and may be connected to theouter conductor of the RF pipe and, thereby connected to ground. Asdescribed in detail below with reference to FIG. 1B, the current sensoris a single conductive loop with two open ends (referred to as ahalf-loop); the loop being completed using, for example, components ofan external V-I analyzer connected to the current sensor by coaxialcables. A V-I analyzer is a measurement system that analyzes the rawsignals received from the current and voltage sensors. As explainedfurther below, both ends of the current sensor may be connected to theV-I analyzer to make a differential measurement for an accurateanalysis. However, to simplify the system at the cost of accuracy, onlyone of the ends of the current sensor may be connected to the V-Ianalyzer and the other end terminated by a load impedance (e.g., by 50ohm load) to ground or shorted to ground. The ground connection may be adirect connection to the sensor casing. In this configuration, the outerground cover of the gallery is in the circuit between the two ends ofthe half-loop contributing significantly to complete the loop. In analternate RF system using embodiments of the current sensors describedin this disclosure, the entire closed loop may be contained within thegallery with appropriate impedance matching and one or more externalsignal connections.

In the example embodiments described with reference to FIGS. 1B-5 , thecurrent loop of the half-loop current sensor has one conductive turncomprising three conducting elements. The three conductive elements ofthe half-loop of the current sensor are two identical vertical branchesconnected by a horizontal branch oriented parallel to the central axisof the RF pipe. Accordingly, the current sensor designs discussed hereinhave mirror symmetry about a mirror plane normal to the central axis ofthe RF pipe and passing half-way between the two vertical branches. Theadvantages of having reflection symmetry have been explained in thediscussion below with reference to FIG. 2B. However, being located onone side of the RF pipe, the single-turn half-loop current sensors arelacking in axial symmetry. Axisymmetric multi-turn half-loop currentsensor designs are described with reference to FIGS. 6A-7E, wherein theembodiments utilize toroidal mandrels to mechanically support themulti-turn current pickups.

The innovative aspects of the V-I sensor designs described in thisdisclosure may provide several advantages. For example, non-intrusiveprobing of the electric and magnetic fields of the electromagnetic wavehas been used to allow performing the V-I measurements with negligibledisturbance of the RF signal in the RF pipe. Also, geometrical symmetryand differential measurement techniques are advantageously utilized inthe sensor designs to provide measurements that may be insensitive tomachining errors due to standard tolerances of the tools used to formthe components as well as positioning errors during assembly of the V-Isensors. In addition, several structural enhancement techniques havebeen utilized; for example, duplicate placement of an element of the V-Isensor may be done to enhance geometrical symmetry, and parts designedto provide extra mechanical support may be placed to reduce/suppresseven small deformations in the shape of critical sensor componentscaused by mechanical stresses during assembly. Thus, by using theembodiments described in this disclosure, RF V-I measurements withimproved precision may be achieved without incurring the increased costof tighter machining tolerance.

FIG. 1A is a block diagram of a generic plasma processing system thatmay be used for semiconductor IC fabrication.

Referring now to FIG. 1A, in a plasma processing system, an RF signalmay be generated by a high power RF power source 10, for example, an RFoscillator coupled to an RF power amplifier. The RF signal waveform(e.g., frequency, amplitude, pulsed/continuous, and the like) may beadjusted by a programmable controller 20 and associated electroniccircuitry. The RF signal may be transmitted via a conduit, e.g., an RFpipe 110, to bring RF power to electrodes coupled to the plasma insidethe block indicated as plasma chamber 30 in FIG. 1A.

As known to persons skilled in the art, the RF signal in the RF pipe 110may be represented as a combination of travelling RF electromagneticwaves. Impedance mismatch between the output impedance of the RF powersource 10 and the load impedance results in a fraction of the RF powertravelling from the RF power source 10 towards the load gettingreflected back to the RF power source 10. In order to suppress suchunwanted reflections, a matcher 40 comprising a matching network may beinserted in the RF signal path between the RF power source 10 and theplasma chamber 30, as illustrated in FIG. 1A. A ratio of reflected powerto incident power of the matching network may be sensed by the matcher40 (e.g., using a V-I sensor and analyzer) and provided to theprogrammable controller 20. The programmable controller 20 may reducethe RF power reflected back to the RF power source 10 from the matchingnetwork by adjusting its impedance using, for example, a feedbackcontrol loop (indicated in FIG. 1A by two arrows between the matcher 40and the programmable controller 20).

Plasma may be sustained in the plasma chamber 30 using, for example, RFpower delivered by RF pipe 110 from the RF power source 10 to anelectrode of the plasma chamber 30. As illustrated in FIG. 1A, a V-Isensor 100 may be used to sense the current and voltage of the RF signalprovided to the electrode. In various plasma chamber designs, theelectrode may be inside the chamber walls, e.g., a disc-shaped electrodein capacitively coupled plasma (CCP) chambers or an antenna outside thechamber walls. For example, in inductively coupled plasma (ICP) chambersthe antenna may be a conductive planar spiral placed above a dielectricwindow, or a conductive helix wound around a dielectric cylinder. Theblock, indicated as plasma chamber 30 in FIG. 1A, includes electrodesand antennas coupled to the plasma. For simplicity, in this disclosure,the term electrode refers to electrode and/or antenna. The plasmachamber 30 comprises at least two electrodes, for example, a topelectrode and a bottom electrode electrically coupled to the plasmabetween them. In some designs, it may be advantageous for the bottomelectrode to also be the substrate holder.

Although the block diagram in FIG. 1A shows the RF pipe 110 deliveringRF power to the plasma chamber 30 from a single RF power source 10,there may be more than one RF power source providing RF power to morethan one electrode. For example, the RF power source 10 may provide RFpower to an electrode (e.g., the top electrode) of the plasma chamber30, and a second RF bias power source may supply RF bias power toanother electrode (e.g., the bottom electrode) of the plasma chamber 30,using respective RF pipes, matcher, and V-I sensor positioned close tothe plasma chamber to sense the voltage and current of the RF signalprovided to the bottom electrode.

In FIG. 1A, the V-I sensor 100 used to sense and measure the current andvoltage of the RF signal close to the electrode receiving the RF signalis connected to a V-I analyzer 60. A V-I analyzer 60 may receive the rawoutput waveforms from the V-I sensor 100, reflective of V(t) and I(t),as indicated by an arrow. The V-I analyzer 60 may be a signal processor,for example, a digital signal processor, that may extract various RFsignal characteristics from the raw waveforms. The various RF signalcharacteristics may include the magnitudes |V|, |I|, the phase angle (Φ)between V and I, and peak RF power |V| |I| cos Φ. In addition, harmonicanalysis may be done to extract multiple frequency components. Themeasured RF signal characteristics may be reflective of the plasmaimpedance and plasma properties such as free electron and ion densitiesand ion/radical flux and energy. The V-I analyzer 60 may bepre-calibrated using, for example, RF calibration signals over a rangeof frequency (e.g., from about 0.4 MHz to about 1 GHz) and power (e.g.,from about 0.015 kW to about 30 kW), standard load impedances (e.g.,short circuit, open circuit, 50 ohms, and the like), and a vectornetwork analyzer (VNA).

As indicated by an arrow in FIG. 1A, various RF signal characteristicsreflective of plasma properties may be provided to the programmablecontroller 20 by the V-I sensor 100 and the V-I analyzer 60 and used,for example, for process monitoring or end-point detection. In addition,the programmable controller 20 may use the received RF measurements forprocess control. As known to a person skilled in the art, the plasmaproperties may be altered by altering the RF signal coupled to theplasma. The programmable controller 20 may control the plasma processusing the information from the RF measurements; for example, byadjusting the settings of the RF power source 10, or by adjusting theimpedance of the matching network of the matcher 40.

In embodiments where the V-I sensor 100 is used to estimate and controlparameters of the plasma, it may be advantageous to locate the V-Isensor 100 close to the plasma chamber 30. The V and I of the RF signalat the electrode location may be estimated from the V and I measured ata different location by a V-I sensor 100 located there. However, theerrors in measurement of V and I at the electrode may increase as thedistance between the electrode and the V-I sensor is increased.Theoretically, the transfer matrix used to transform the sensor signalsbetween two locations deviates further from the unity matrix as thedistance between the two locations increase. Accordingly, the V and Iestimated for the electrode location become increasingly sensitive toany error in estimating the respective transfer matrix.

Referring now to FIG. 1B, a V-I sensor 100 is attached to an RF pipe 110that connects to a plasma chamber 30, in accordance with one embodiment.The RF pipe 110 may be a coaxial structure comprising two conductivetubes (e.g., aluminum or copper tubes) placed concentrically about ashared longitudinal axis. The inner conductive tube, referred to as theinner conductor 120, may be electrically connected to the outputterminal of matcher 40 indicated by an arrow pointing to the left. Theouter conductive tube, referred to as the outer conductor 130, may be agrounded sheath connected to a reference potential, generally referredto as ground. The RF pipe 110 may be referred to as the main coaxialline since it carries the RF power from the matcher 40 to the plasmachamber 30. Other coaxial lines in this disclosure are referred to ascoaxial signal lines (e.g., coaxial lines which may be used to carrysignals from the V-I sensor 100 to the VI analyzer 60).

The V-I sensor 100 comprises two primary components: a current sensor140 and a voltage sensor 150. The current sensor 140 may be disposed inan annular gallery 160 inside a sensor casing 165 with conductive walls(e.g., aluminum, brass, stainless steel, or copper). In the embodimentshown schematically in FIG. 1B, the gallery 160 is a hollow annularregion which follows a complete circumference outside of the outerconductor 130 and is axisymmetric about the axis of the RF pipe 110. Theaxisymmetric design of the gallery 160 provides the advantage ofpreventing additional reflections and non-axisymmetric wave modes of theRF electromagnetic waves propagating in the RF pipe 110. The gallery 160and the conductive sensor casing 165 may be formed either integrallywith the RF pipe 110 or be able to be symmetrically attached around theRF pipe 110 and positioned during assembly to help avoid alignmenterrors between the longitudinal axis of the RF pipe 110 and the currentsensor 140. In either case, the conductive sensor casing 165 and theouter conductor 130 are electrically and physically connected.Accordingly, the sensor casing 165 may be considered to be an extensionof the outer conductor 130 of the coaxial RF pipe 110.

Although the embodiment in FIG. 1B has an annular gallery 160, in someother embodiment the gallery may not be annular. In some otherembodiment, the axisymmetry of the RF pipe 110 may have been unavoidablybroken by, for example, bends in the RF pipe 110 and, as such,additional loss of axisymmetry due to an asymmetry in the V-I sensordesign may be insignificant. It may then be reasonable to relax theaxisymmetry in the V-I sensor design. For example, the gallery mayfollow the circumference of the outer conductor 130 partially and notmake a complete circuit of the RF pipe 110.

Referring to FIGS. 1B and 1C, the gallery 160 is shown completelyenclosed by a conductive surface of the sensor casing 165 and the outerconductor 130 except for a slit 132 connecting the hollow regions of thegallery 160 and the RF pipe 110. A current pickup 141 of the currentsensor 140 is shown located in the gallery 160 directly above the slit132. In the embodiment illustrated in FIG. 1B, the current pickup 141comprises three conductive branches arranged as three sides of arectangle (referred to as a half-loop): two vertical branches 142 and ahorizontal branch 143. In one embodiment, the two vertical branches 142are each screwed into an opening in the horizontal branch 143.

In the embodiment in FIG. 1B, the vertical and horizontal branches ofthe current pickup 141 are formed using three separate parts. In someother embodiment, a different number of parts (less/more) may be used.

The slit 132 is designed to permit the magnetic flux to penetrate intothe gallery 160. Current flowing in the inner conductor 120 results inmagnetic flux circulating around the inner conductor 120 about thelongitudinal axis LA1 in the region between the inner conductor 120 andthe outer conductor 130. Without the slit 132, the magnetic flux outsidethe outer conductor 130 would be roughly zero because an equal butopposite return current flowing on the inner surface of the outerconductor 130 would cancel out the circulating magnetic flux due to thecurrent in the inner conductor 120, in accordance with Ampere's law. Theslit 132 diverts the return current to flow along the inner surface ofthe outer conductive body of the sensor casing 165 by breaking thecontinuity in the cylindrical outer conductor 130. Thereby, the hollowregion of the gallery 160 containing the half-loop current pickup 141falls inside the region between the current flowing in the innerconductor 120 and the respective return current. By Ampere's law, thereis now a magnetic field inside the gallery 160 threading through therectangular half-loop of the current pickup 141. In one example, theslit 132 may extend along the entire circumference of the cylindricalouter conductor 130 to help maximize the magnetic flux which threads thehalf-loop of the current pickup 141.

In addition to the magnetic flux, there is electric flux emanating fromthe inner conductor 120 due to a voltage difference between the innerconductor 120 and the grounded outer conductor 130. Unwanted electricflux may leak into the gallery 160 through a gap in the grounded sheathprovided by the slit 132 made in the outer conductor 130. The changingmagnetic flux threading through the half-loop of the current pickup 141induces an electrical signal that is a measure of I(t) at that location.However, electric flux entering the gallery 160 may couple with thecurrent pickup 141 and contaminate the signal produced by the magneticflux. Accordingly, as illustrated in FIG. 1C, slit 132 has been designedto have a width (dimension parallel to LA1) about 1 mm to about 5 mm.The width of the slit 132 may be kept narrow to help reduce the electricflux entering gallery 160 from inside the RF pipe 110.

Although the slit design used for the embodiments of V-I sensorsdescribed with reference to FIGS. 1B-5 are shaped like a ring along thecircumference of the outer conductor, it is understood that variousother designs are possible. For example, a zig-zag slit design has beenused in a current sensor assembly described with reference to FIG.7A-7E.

The conductive parts of the current sensor 140 may be insulated from theconductive surfaces of the outer conductor 130 and the sensor casing 165by air gaps (or other insulators) and by insulating components used formechanical support, such as the insulating parts 162 in FIG. 1 b (andsimilar other parts shown in FIGS. 2A-5 ).

The current pickup 141 is topologically a half-loop (a loop with twoopen ends) making one turn around a region with a rectangularcross-section with its two vertical branches 142 and a horizontal branch143. The single-turn half-loop current pickup 141 may be positioned inpresence of a time-varying magnetic field originating from RFelectromagnetic waves traveling along the RF pipe 110. By Faraday's law,a time-varying voltage difference may be induced between the two ends ofthe current pickup 141 proportional to the time-varying magnetic flux.The two ends of the current pickup 141 may be attached to a symmetricpair of terminals 144 shown above the sensor casing 165 in FIG. 1B. Inone embodiment, the terminals 144 may be coaxial cable connectors usedto connect coaxial signal lines.

As explained in further detail below, it is advantageous to use asymmetric design for the current pickup 141. The symmetry is utilized bya measuring system (e.g., the V-I analyzer 60 in FIGS. 1A and 1B) tocancel out parasitic signals in the two vertical branches 142 bymeasuring, for example, the differential voltage between the twoterminals 144 of the current sensor 140. For this measurement method,the differential signal from the current sensor 140 is its outputsignal, and may be detected using, for example, a differentialamplifier.

The half-loop of the current pickup 141 is completed external to the V-Isensor by a combination of terminating impedances, input impedances ofan initial detection system, and impedances of cables (if cables areused to transmit the output signal of the current sensor 140 to theinitial detection system of the measuring system). If the initialdetection system is placed at the current sensor 140 itself then therequirement of matching the detector impedance to cable impedances maybe lifted. If the measuring system is remote from the current sensor 140then the terminals 144 may be connected to the initial detection systemof the measuring system using coaxial signal lines comprising, forexample, coaxial cables. Coaxial cables have impedances typically in therange of about 20 ohms to about 300 ohms. It is advantageous toterminate the coaxial signal lines with matching impedances in order toavoid reflections from the measuring system due to the impedancemismatch. It is also advantageous to connect the two end terminals 144of the current sensor 140 to symmetric coaxial signal lines terminatedin a symmetric fashion, thereby preserving the symmetry of the outputsignals of the current sensor 140. For example, in one embodiment, apair of identical 50 ohm coaxial cables with 50 ohm terminations may beused.

As mentioned above, use of the differential signal as the output signalof the current sensor provides higher accuracy in measuring I. In orderto detect the differential signal, the pair of signals from the pair ofterminals 144 has to be provided to the initial detection system using,for example, a pair of coaxial cables. However, with some loss inaccuracy, the current sensor may also be used in conjunction with ameasuring system that detects the signal at one of the terminals of thepair of terminals 144. In systems that detect the signal at a firstterminal of the pair of terminals 144 (instead of detecting thedifferential signal), the second terminal of the pair of terminals 144may be connected to impedances that reflect the impedances at the firstterminal as closely as possible. For example, the first terminal may beconnected to a first 50 ohm coaxial cable to transmit the signal to a 50ohm input port of an initial detection system, and the second terminalmay be connected to a second identical 50 ohm coaxial cable having a 50ohm termination at the end of the cable instead of a detector. It shouldbe noted that some other appropriate impedance may be used fortermination; the impedance is not required to be 50 ohms. Furthermore,the second coaxial cable may be omitted and the appropriate impedancetermination be affixed directly to the second terminal of the pair ofterminals 144.

The design of the measuring system, including the elements used toconnect the current sensor 140, also takes into consideration impedancesto ground due to parasitic capacitances of the electronic components.Because of the frequency dependence of the parasitic capacitiveimpedances, the impedance of a component at RF frequencies may differsignificantly from the component's impedance at a low frequency or at DC(zero frequency). For example, the impedance of a resistor component atDC may get reduced as the frequency of the electrical signal isincreased to the RF range because of parasitic capacitance to groundassociated with the resistor structure. The impedance of a resistorhaving a higher value of resistance is more sensitive to the frequencyof the RF signal. Since the parasitic capacitance to ground depends onthe geometry and the geometrical environment in which the resistor isplaced, it is difficult to control the variations in impedance from unitto unit if the resistor has a high value of resistance. Thus, in orderto maintain measurement accuracy, it is advantageous to restrict thedesign of the initial detection system to using resistors whoseresistance value is less than 0.1 of its parasitic RF reactance, evenwhen the initial detection system is placed at the sensor location.

A voltage pickup 151 of the voltage sensor 150 comprises a conductivering that may be placed along the inner surface of the outer conductor130. The outer conductor 130 and the conductive voltage pickup 151 maybe insulated from each other by an insulating ring 152, as illustratedin FIG. 1B. The insulating ring 152 may comprise Teflon, or some otherplastic materials, or some other suitable dielectric. In one embodiment,the voltage pickup 151, such as the conducting ring, may be exposed tothe air (or other insulator) between the inner conductor 120 and theouter conductor 130. In another embodiment, voltage pickup 151 may beembedded in an insulating housing. In all embodiments, the voltagepickup 151 (e.g., the conducting ring) may be electrically insulatedfrom the outer conductor 130 and mechanically supported by an insulatingstructure.

In one embodiment, the inner diameter of the voltage pickup 151 (e.g.,the conducting ring) may be the same as the inner diameter of the outerconductor 130. In some other embodiment, the inner diameter of thevoltage pickup 151 (e.g., the conducting ring) may be different (smalleror larger than the inner diameter of the outer conductor 130). Theperturbation to the electric and magnetic fields in the RF pipe 110caused by inserting the voltage pickup 151 is relatively the lowest whenthe inner diameter of the voltage pickup 151 (e.g., the conducting ring)and the inner diameter of the outer conductor 130 are equal. The outputsignal from the voltage pickup 151 increases as the inner diameter ofthe voltage pickup 151 (e.g., the conducting ring) decreases, asexplained in further detail below. A contact to the voltage pickup 151extends outside the outer conductor 130 and terminates at a thirdterminal 153 (e.g., a third coaxial cable connector) attached above thesensor casing 165. The current pickup 141, the voltage pickup 151, andthe contacts to the respective terminals 144 and 153 may comprise ametal (e.g., copper) with high electrical conductivity, and may all beinsulated from other conductive elements such as the outer conductor 130and the conductive sensor casing 165.

The considerations for the design for the termination impedance andcoaxial signal line connecting an initial detection system to theterminal 153 of the voltage sensor 150 may be similar to that for thetermination impedances and coaxial signal lines connecting an initialdetection system to the terminals 144 of the current sensor 140, asdiscussed above. The discussion above, with reference to the currentsensor 140, includes considerations for preserving symmetry of thedifferential output signal. However, that portion of the discussion isnot applicable to the voltage sensor because, in the embodiment of theV-I sensor 100, the voltage sensor 150 has only one ring-shaped voltagepickup 151 and one terminal 153, whereas the current sensor 140 has apair of terminals 144. Symmetry considerations may be applicable inanother embodiment, where two voltage pickup rings are placedsymmetrically and an arithmetic mean of the two signals may be used, forexample, in the V-I sensor 300, described with reference to FIG. 3 .

As illustrated in FIGS. 1B and 1C, the longitudinal axis LA1 of the RFpipe 110 is in the plane P1 of the current pickup 141. The longitudinalaxis LA1 is also parallel to the direction of the current in the RF pipe110. In addition, as is more easily observed in FIG. 1C, along adirection orthogonal to the longitudinal axis LA1 of the RF pipe 110,the current pickup 141 comprises a first plane of mirror symmetry M1comprising the longitudinal axis LA1 of the RF pipe 110 and a secondplane of mirror symmetry M2 orthogonal to the first plane of mirrorsymmetry M1. The first plane of mirror symmetry M1 of the current pickup141 and the longitudinal axis LA1 of the RF pipe 110 are co-planar inone or more embodiments.

The magnetic field lines are roughly concentric around the longitudinalaxis LA1, passing perpendicularly through the plane P1 of the half-loop.In this configuration, the magnetic field gets inductively coupled tothe current pickup 141 (as is desired). Undesirable coupling to theelectric field is greatly weakened by locating the current pickup 141outside the outer conductor 130. The inductively coupled oscillatingmagnetic field induces an electromotive force (emf) in the currentpickup 141 (the three-sided half-loop). The induced emf is related tothe changing magnetic flux, in accordance with Faraday's law. Since thestrength of a magnetic field around a current-carrying conductor isreflective of the respective electrical current, the current sensor 140may generate a time-varying electrical signal reflective of the RFcurrent in the RF pipe 110 at the respective location. An aspect of thecurrent sensor 140 is that that the electrical signal at both of theterminals 144 may be received by the detection system, and thedifferential voltage between the two terminals 144 be used as the outputsignal of the current sensor 140. The advantage provided by thedifferential output technique is explained in further detail below withreference to FIG. 2B.

The electric potential and electric field magnitude contours are roughlycircles with their centers on the longitudinal axis LA1 of the RF pipe110. The circular contours are contained in the family of planes normalto the longitudinal axis LA1. Thus, the electric field lines areradially directed from the inner conductor 120, perpendicular to thelongitudinal axis LA1. The ring-shaped voltage pickup 151 is locatedroughly on one of the circular contours. For this configuration, theoscillating electric field in the space outside the inner conductor 120is capacitively coupled to the voltage pickup 151, and the conductivering attains an oscillating electric potential roughly proportional tothe electric potential of the inner conductor at the respectiveposition, in accordance with the physical laws of electromagnetism. Thisoscillating electric potential may be used as the output signal of thevoltage sensor 150. The magnitude of the radial electric field betweenthe inner conductor 120 and the outer conductor 130 decreases withincreasing radial distance from the longitudinal axis LA1, in accordancewith Gauss' law. Accordingly, the output signal of the voltage sensor150 may be increased by positioning its voltage pickup 151 closer to theinner conductor 120, for example, by reducing the inner diameter of thevoltage pickup ring.

While the voltage pickup 151 is capacitively coupled to the electricfield, there is almost no coupling with the magnetic field because themagnetic flux normal to the plane of the ring-shaped voltage pickup 151is negligible for this geometry. Since the strength of the electricfield around a conductive tube (the inner conductor 120 in this example)is reflective of the electric potential of the conductor, the voltagesensor 150 may generate a time-varying electrical signal reflective ofthe RF voltage on the RF pipe 110 at the respective location.

The raw output signals (e.g., one pair from the current sensor 140 andanother from the voltage sensor 150) may be transmitted to a V-Ianalyzer 60, as indicated by arrows (see also FIG. 1A).

FIG. 2A illustrates a perspective view of a V-I sensor 200 and the outerconductor 230 (the outer tube) of an RF pipe. FIG. 2B illustrates acutaway view of the same V-I sensor 200 along an axis 2B-2B′. The V-Isensor 200 in FIGS. 2A and 2B is similar to the V-I sensor 100 in FIG.1B. The current sensor 240 and the voltage sensor 250 are placed withinthe annular gallery 260 of the V-I sensor 200. In FIGS. 2A and 2B, theinner conductor of the RF pipe has been removed to better illustrate thevoltage pickup ring 251 of the voltage sensor 250 inside the outerconductor 230. The perspective view (FIG. 2A) shows three terminals(coaxial cable connectors in this example) of the V-I sensor 200. Thepair of terminals 244 extending above the top of the gallery 260 connectto the current pickup 241 of the current sensor 240, as seen in FIG. 2B.The third terminal 253 connects to the voltage pickup 251 of the voltagesensor 250.

Referring to FIG. 2B, the current pickup 241 of the current sensor 240is a half-loop comprising three conductors. Two conductive verticalbranches 242 of the current pickup 241 are insulated by plastic (orother insulating materials) from the metallic sensor casing 265. Thevertical branches 242 connect to two ends of a horizontal branch 243,which is a third conductor disposed horizontally inside the gallery 260above the outer conductor 230. A slit 232 along a circumference of theouter conductor 230 allows the magnetic field to thread through theplane of the half-loop and induce an electromotive force in theconductive branches of the current pickup 241. The horizontal branch 243of the current pickup 241 may be attached to a horizontal non-conductive(e.g., plastic) parts 262 along the side of the conductor. In theexample embodiment of the V-I sensor 200 illustrated in FIGS. 2A and 2B,the horizontal branch 243 is insulated from the grounded metallic sensorcasing 265 and the outer conductor 230 by plastic parts 262 on the sidesand by air in the gap between the bottom of the horizontal branch 243and the top of the outer conductor 230. In another embodiment, describedin further detail below, the mechanical support for the horizontalbranch 243 may be strengthened by additional plastic parts placed in theair gap below the horizontal conductor.

The current pickup (e.g., the current pickup 240 in FIG. 2B) provides anelectrical signal through its interaction with the RF electromagneticfields. As explained above, it is the magnetic field (not the electricfield) that is reflective of the RF current. A slit 232 allowspenetration of the magnetic field from the RF pipe into the gallery 260in which the current pickup 241 is located. Any coupling of the currentpickup 241 to the electric field degrades the precision in themeasurement of the magnetic field. The current sensor 240 may suppressthe measurement error that may arise from unwanted interaction with theelectric field, as explained herein. First, the current sensor 240 inthe V-I sensor 200 is placed outside the grounded outer conductor 230,thereby using the outer conductor 230 to shield the electric field. Asmentioned above with reference to FIGS. 1B and 1C, the RF electric fieldis in the radial direction (perpendicular to the coaxial axis LA1 of theRF pipe and, accordingly, the electric flux leaking into the gallery isroughly directly proportional to the slit width, defined above as thedimension parallel to LA1. The width of slit 232 may be selected to berelatively small to reduce the amount of electric flux entering thegallery 260 because of the gap created by slit 232 in the outerconductor 230. Second, a differential signal may be used as the outputsignal in order to further reduce the impact of the fraction of theelectric field that could penetrate into the cavity despite the outerconductor 230. Ideally, the differential voltage between the twoterminals 244 of the current sensor is roughly proportional to theoscillating magnetic field, in accordance with the theory ofelectromagnetism. But, because of the presence of the slit 232, a weakelectric field inside the gallery 260 may get capacitively coupled tothe current pickup 241. However, the current pickup 241, slit 232, andgallery 260 may be constructed to be mirror symmetric about a planewhich passes through the center of the slit 232 and is orientedperpendicular to the longitudinal axis of the RF pipe. Because of thisgeometrical mirror symmetry of the half-loop current pickup 241mentioned above, the perturbations along the vertical branches 242 andon the electric potentials appearing at the two terminals 244 areroughly equal in magnitude and phase. This symmetry property may be usedadvantageously because it implies that the differential signal is immuneto the parasitic signals in the current pickup 241 due to interactionwith the penetrating electric field in the gallery 26 o. In other words,the potential difference between the first terminal and the secondterminal of the terminal pair 244 remains unperturbed, correct to firstorder precision. These aspects of the current sensor 240 design may beused advantageously to achieve measurements of current with highprecision, particularly in applications such as providing an RF biassignal to an electrostatic substrate holder in a plasma chamber, whereinthe load impedance may be such that the amplitude of the electric fieldis relatively high and the amplitude of the magnetic field is relativelylow close to the point where the RF signal enters the plasma chamber.

Still referring to FIG. 2B, the ring-shaped conductor placed inside theouter conductor 230 close to its inner surface is the voltage pickup 251of the voltage sensor 250. The strength of the signal generated by thevoltage pickup 251 may depend on its dimensions. While the diameter maybe roughly determined by the diameter of the outer conductor 230, thewidth and thickness are adjustable design parameters. The conductivevoltage pickup 251 in this embodiment is a ring electrically connectedat one point to the third terminal 253 (e.g., a coaxial cable connector)of the V-I sensor 200. The conductive voltage pickup 251 is insulatedfrom the conductive outer conductor 230 by a ring-shaped dielectriccomponent 252 attached to the voltage pickup 251.

As explained above, the voltage pickup 251 provides an electrical signalat the third terminal 253 of the V-I sensor 200 resulting from chargepolarization induced by the RF electromagnetic field. The electricpotential at the third terminal 253 is reflective of the oscillatingvoltage of the inner conductor (not shown in FIG. 2B for clarity). TheRF electric field gets capacitively coupled to the voltage pickup 251.However, the interaction with the magnetic field is negligible becausethe longitudinal axis is normal to the plane of the ring, as explainedabove with reference to FIG. 1B.

The ring-shaped design of the voltage pickup 251 uses axial symmetry toreduce the sensitivity of the output of the voltage sensor 250 to someof the placement and sizing errors, as explained herein. First, thecircular symmetry may remove the need for precise placement of thevoltage pickup 251 because, to first order precision, the electricpotential at the conductive surface of the ring is independent of theoffset in the location of the ring's center from the central axis (axis2B-2B′ in FIG. 2A and LA1 in FIG. 1B). Rather, the potential of thevoltage pickup 251 depends primarily on the dimensions of the ring(e.g., the inner diameter, outer diameter, and thickness). In contrast,in an asymmetrical voltage pickup design, the voltage pickup conductorwould acquire an electric potential that, to first order precision,depends on the size as well as the placement of the asymmetrical voltagepickup. For example, a mushroom-shaped voltage pickup may be sensitivenot only to the dimensions of the conductive surface at the mushroomhead but also to its position relative to the inner conductor. In suchdesigns, the distance between the voltage pickup and the longitudinalaxis of the RF pipe may have to be adjusted precisely during assembly,sometimes manually using a micrometer screw gauge. Second, the design ofthe voltage pickup 251 may roughly cancel out any centering error duringassembly of the voltage sensor 250 into the V-I sensor 200. Axialsymmetry of the voltage pickup 251 ensures that, to first orderprecision, the total electric flux is unaltered even if the center ofthe circle of the voltage pickup 251 is slightly displaced from thelongitudinal axis of the inner conductor. The increase in electric fluxin one half of the ring that may be displaced closer to the innerconductor is balanced by a concomitant decrease in the electric flux inthe other half of the conductive ring that would now be further from thelongitudinal axis because of the circular geometry of a ring.

FIG. 3 illustrates a cutaway view of another embodiment of a V-I sensor300 having a current sensor 340 located in a gallery 360 inside a sensorcasing 365. The horizontal branch 343 is shown supported by insulatingparts 362 and connected to the two vertical branches 342 of the currentsensor 340. A slit 332 going around in the plane of mirror symmetry M2is seen above the horizontal branch 343.

A first voltage sensor 350 is shown inside an outer conductor 330,similar to the V-I sensor 200 described with reference to FIGS. 2A and2B. In addition, the V-I sensor 300 has a second voltage sensor 355located symmetrically on the opposite side of the current sensor 340. Inthis embodiment, the voltage pickup and housing of the first and secondvoltage sensors 350 and 355 have been recessed into the body of theouter conductor 330 in order to keep the inside surface of the outerconductor 330 as smooth as possible. The smooth inner surface of theouter conductor 330 provides the advantage of reducing the perturbationto the electromagnetic fields caused by inserting the voltage sensors350 and 355. In this embodiment, the V-I sensor 300 causes negligibleperturbation to the electromagnetic fields in the RF pipe.

The measurements from the first voltage sensor 350 and the currentsensor 340 have a relative phase error because of the difference inmeasurement locations between them. In this embodiment, an oppositelyplaced second voltage sensor 355 has an opposite phase error because ofthis symmetric location relative to the current sensor 340 (i.e., theplane of mirror symmetry M2 of the current sensor 340 is equidistantfrom first voltage sensor 350 and the second voltage sensor 355). Fromsymmetry, the error in the relative phase between voltage and current inthe RF signal waveform sensed by the first voltage sensor 350 and therespective error in the RF signal waveform sensed by the second voltagesensor 355 cancels out in the sum of the two sensed voltage signals, atleast to first order precision. Accordingly, a more accurate voltagemeasurement may be provided by combining the signals from the first andsecond voltage sensors 350 and 355. By using, e.g., the arithmetic meanof the measurements from the first voltage sensor 350 and the secondvoltage sensor 355, the phase error may be reduced or even eliminated toyield a voltage measurement reflective of the voltage at the plane ofmirror symmetry.

In addition, the presence of the second voltage sensor 355 helps ensurethat the two vertical branches 342, as well as the left half and righthalf of the horizontal branch 343 of the current pickup 341 experiencethe same electric and magnetic fields. As explained above, parasiticelectrical signals may be generated by unwanted coupling of the currentpickup to the electric field penetrating into the cavity in the gallery360. By improving the geometrical symmetry, the second voltage sensor355 helps ensure that the perturbations in the electric potentials seenat the first and second terminals 344 of the current sensor 340 getcancelled out more accurately by using the differential currentmeasurement, as described above with reference to FIGS. 2A and 2B. Insome embodiments, it may be optional to use the output of the secondvoltage sensor 355.

FIG. 4 illustrates yet another embodiment of a V-I sensor 400 attachedto an RF pipe 410 comprising an inner conductor 420 and an outerconductor 430.

The V-I sensor 400 comprises a current sensor 440 and a voltage sensor450 placed in a gallery 460, similar to the V-I sensor 200 describedwith reference to FIGS. 2A and 2B. The design of the V-I sensor 400 hasbeen improved relative to the design of the V-I sensor 200 (see FIGS. 2Aand 2B) by providing additional mechanical support to the horizontalconductor 443 of the current pickup 441 of the current sensor 440.

In the design of the V-I sensor 400, illustrated in FIG. 4 , thesupporting components (e.g., plastic parts 462 and 470) may fix thehorizontal branch 443 more robustly than the respective parts (e.g.,plastic parts 262) in the V-I sensor 200, illustrated in FIG. 2B. Forexample, in one embodiment, the plastic parts 262 at the two ends of thehorizontal branch 243 in V-I sensor 200 are rings with a set of holesinto which the horizontal branch 243 may be placed, whereas, in thedesign of the V-I sensor 400, the plastic parts, such as the parts 462and 470, encompass more of the horizontal branch 443, and may havebosses which fit tightly into matching cavities in the metal sensorcasing 465 and metallic outer surface of the outer conductor 430.

As illustrated in FIG. 4 , supporting structures 470 (e.g., made ofplastic or other non-conductive materials), placed in addition to theinsulating supports 462, hold the conductive horizontal branch 443 ofthe current pickup 441 from all sides. The supporting structures 470include a first portion for supporting a first part of the horizontalbranch and a second portion for supporting a second part of thehorizontal branch and are separated by a gap. In contrast, asillustrated in FIG. 2B, the plastic parts 262 (similar to the supports462) do not support the horizontal branch from below. In FIG. 2B, thereis empty space between the horizontal branch 243 of the current pickup241 of the current sensor 240 and the outer conductor 230. The extrasupport prevents the horizontal conductor 443 from bending when thevertical branches 442 of the current pickup 441 are placed in contactwith the horizontal branch 443. In addition, the supporting structures470 may prevent over tightening of the vertical branches 442 to thehorizontal branch 443. The variations in the magnetic flux coupled tothe current pickup 441 are influenced by the variations in the shape andarea of the half-loop geometry of the current pickup 441. Hence,stabilizing the shape of the current pickup 441 reduces variations inthe electrical output of the current sensor 440 and improves theprecision of the measurement of current.

FIG. 5 illustrates a V-I sensor 500 attached to an RF pipe 510comprising an inner conductor 520 and an outer conductor 530. A currentsensor 540 is shown having a pair of terminals 544 disposed over thesensor casing 565 and a single-turn half-loop current pickup 541disposed inside a gallery 560. The current pickup 541 comprises twovertical branches 542 attached to a horizontal branch 543. Similar tothe V-I sensor 400 in FIG. 4 , the plastic parts 570 have been used toprevent over tightening of the vertical branches 542 and bending of thehorizontal branch 543 of the current pickup 541 during assembly of thecurrent sensor 540.

The V-I sensor 500 includes improvements that reduce the machiningcomplexity, thereby reducing manufacturing cost relative to the V-Isensor 400 (shown in FIG. 4 ). The design of V-I sensor 500 has beenimproved over that for V-I sensor 400 by using a voltage sensor 550 inwhich the insulator pieces 555 which center the inner conductor 520 ofthe RF pipe 510 are also used to support the conductive voltage pickupring of the voltage sensor 550, as illustrated in FIG. 5 . Using thesame plastic parts 555 for multiple purposes allows elimination of someof the plastic parts used, e.g., in the V-I sensor 400. This reduces themachining complexity and manufacturing cost for the V-I sensor 500.

The conductive voltage pickup ring of the voltage sensor 550 in FIG. 5has been positioned closer to the inner conductor 520 by designing thediameter of the voltage pickup ring to be smaller than the diameter ofthe outer conductor 530. The smaller diameter of the voltage pickup ringincreases the output signal strength of the voltage sensor 550, asexplained above with reference to FIGS. 1B and 1C.

Although a single turn half-loop current pickup has been used in the V-Isensors described above with reference to FIGS. 1-5 , it is understoodthat multiple turns may also be utilized in the design of a currentpickup of the current sensor. For example, the current pickup in the V-Isensors illustrated in FIGS. 1-5 may comprise a plurality of rectangularturns between the two ends of the current pickup connected to the twoterminals of the current sensor. As mentioned above, multi-turn currentpickups may also be constructed by winding a conductive wire around amandrel, for example, a toroidal mandrel. The conductive wire may bewound in a coil about the circular axis of a doughnut shaped insulatingmaterial that circles symmetrically around the inner conductor of an RFpipe that passes perpendicularly through the central hole of the toroid.Multi-turn current pickups using a toroidal mandrel are described belowwith reference to FIGS. 6A-7E.

It is understood that the mandrel may not conform exactly to themathematical definition of toroid, but that it is generally shaped likea toroid with structures to attach a coil, make connections toterminals, and the like.

FIG. 6A illustrates a perspective view of a V-I sensor 600, FIG. 6Billustrates a cutaway view, and FIG. 6C illustrates a cross-sectionalview along an axis A-A′ of the V-I sensor 600.

FIG. 6A shows a conductive sensor casing 665 of the V-I sensor 600. Thecurrent and voltage pickups, not visible in FIG. 6A, are housed within aspace enclosed by the conductive sensor casing 665. In FIGS. 6A-6C theinner conductor would pass through a central hole 621. The innerconductor itself is omitted from the various views of the V-I sensor 600in FIGS. 6A-6C for clarity. The outer conductor of the RF pipe wouldconnect to flanges at the top and the bottom of the conductive sensorcasing 665. Two neck regions 631 are shown adjacent to the flanges atthe top and bottom portions of the sensor casing 665 in FIG. 6A. Theshape and dimension of the neck region 631 may be designed to be ofsimilar to that of the outer conductor of the RF-pipe. The sensor casing665 may thus be construed as an extension of the outer conductor thatexpand from the neck regions 631 into a wider central portion comprisinga top cover 663 and a bottom cover 666 having conductive walls of alarger diameter. The sensor casing 665 and the outer conductor form theouter shield of the coaxial structure and may be connected to ground. Asdescribed below with reference to FIGS. 6B and 6C, the wider centralportion of the sensor casing 665 accommodates an annular dielectriccavity 661 around the inner conductor passing through the central hole621.

The perspective view in FIG. 6A also shows three coaxial cableconnectors assembled outside the sensor casing 665. The three coaxialcable connectors are three terminals of the V-I sensor 600. The outsidepair of coaxial connectors 645 connects to the terminals of the currentsensor 641, connected to the current pickup, and the coaxial connectorin the middle connects to the center terminal 654, connected to thevoltage pickup 651 of the voltage sensor. The current and voltagepickups are located between the top cover 663 and bottom cover 666.

The cutaway diagram in FIG. 6B and the cross-sectional view in FIG. 6Calong a cutting plane A-A′ (shown in FIG. 6A) illustrate the internalstructure of the V-I sensor 600. The inner conductor of the RF pipe isomitted for clarity. The inner surface 638 of the central hole 621, asshown inside the neck region 631 in FIGS. 6B and 6C, forms the sidewallinside the main coaxial structure which may be construed as the RF pipe.The neck region 631 expands into the wider diameter top and bottomcovers 663 and 666 of the sensor casing 665 enclosing the dielectriccavity 661 in a section around the central hole 621 of the V-I sensorassembly 600. The inner wall 638 continues as the surface 627 of the topand bottom covers 663 and 666 till it is interrupted by the slit 671. Asillustrated in FIG. 6C, the dielectric cavity 661 is between a firstmajor outer surface 627 and a second major outer surface 628 along aradial direction frog a center of the central hole 621. The first majorouter surface 627 comprises a continuously circular ring shaped regionin physical contact with the central hole 621. Vertically, the ringshaped first major outer surface 627 is separated into two parts by theslit region 671. The second major outer surface 628 is located at aradial distance greater than a radius of the first major outer surface627.

The cavity 661 includes a slit region 671. As illustrated in FIGS. 6Band 6C, the slit region 671 comprises a physical break in the innersurface 627 of the cylindrical wall of the central hole 621 that forms agap in the juncture between the top and bottom covers 663 and 666 of thesensor casing 665. Surface 627 is an extension of surface 638 of thecylindrical wall of the central hole 621. When viewed radially from thecenter of the central hole 621, the slit region 671 has the appearanceof an insulating ring in physical contact with the central hole 621.Further radially outwards, the slit region 671 takes a zig-zag shapethat circumvents the ring-shaped conductive voltage pickup 651, asindicated by zig-zag dashed lines in FIGS. 6B and 6C. The continuousinsulating annular region of cavity 661 radially disposed between thephysical contact with the central hole 621 and the inner radius of thetoroidal current sensor 641 is referred to as the slit region 671 of theV-I sensor 600. As illustrated in FIGS. 6B and 6C, the slit region 671forms a dielectric barrier interposed between the conductive voltagepickup 651 and the current sensor 641, as well as the sensor casing 665.In the region radially between the first major outer surface 627 andsecond major outer surface 628, the cavity 661 (which includes theinsulating slit region 671) electrically isolates the top cover 663 andthe bottom cover 666. For radial distances smaller than the first majorouter surface 627, the top cover 663 is electrically isolated from thebottom cover 666 by the central hole 621. The top cover 663 iselectrically coupled to the bottom cover 666 through a coupling region629 radially beyond the second major outer surface 628 of the dielectriccavity 661.

With this design, almost none of the RF current flowing in the groundedsensor casing 665 may flow in the region surrounded by the toroidalcurrent sensor 641. The current would flow vertically in the neck region631 along the inner wall 638 and then be routed around the currentsensor 641 because of the physical break in the inner surface 638created by the dielectric slit 671. On account of the slit 671, thecurrent would be diverted radially outwards around the toroidal currentsensor 641, flowing laterally along the conductive walls of the annulardielectric cavity 661, return back in radially, and then continuevertically along the inner wall 638 of the neck region 631.

Referring to FIGS. 6B and 6C, the current sensor 641 is the toroidalstructure inside the annular dielectric cavity 661, in the outer portionof the cavity, i.e., the region farther from the central hole 621. Thecurrent sensor 641 comprises a conductive coil 647 and a toroidalmandrel 642. The coil 647 comprises a plurality of turns of a continuousconductive wire wound around the central circular axis of the toroidalmandrel 642. The two opposite ends of the coil 647 may be attached tothe coaxial connectors 645, as illustrated in FIG. 6B. The conductivewire of coil 647 may be a bare conductor, an enameled conductor, or aconductor coated with an insulator. The toroidal mandrel 642 isdescribed in further detail below with reference to FIG. 6D. The currentsensor 641 is electrically insulated from the conductive sensor casing665.

As illustrated in FIGS. 6B and 6C, the conductive voltage pickup 651 ofthe voltage sensor is shaped like a conductive ring. The voltage pickup651 is shown disposed in the region of the annular dielectric cavity 661between the toroidal current sensor 641 and the central hole 621. Soliddielectric material (e.g., plastic) may be used for parts used toprovide mechanical support for the voltage pickup 651. The slit region671 of the cavity 661 and the dielectric supporting parts electricallyisolate the conductive voltage pickup 651 from the conductive sensorcasing 665. The connection between the voltage pickup 651 and the centerterminal 654 is illustrated in FIG. 6B. (The coaxial connectors 645 andthe center terminal 654 are not included in the cutting plane A-A′;hence not visible in the cross-sectional view shown in FIG. 6C.)

The function of the voltage pickup 651 is to sense the RF voltage of theinner conductor at the center of the central hole by sensing a radialelectric field between the inner conductor and the outer conductor ofthe RF pipe. Generally, the outer conductor of the RF pipe and theconductive sensor casing 665 are grounded. Accordingly, the voltagepickup 651 may not be able to function properly if the conductive ringof the voltage pickup 651 gets shielded from the inner conductor of theRF pipe by, for example, a grounded metal ring placed in the annulardielectric region between the inner conductor and the voltage pickup651. With excessive shielding the voltage sensor output would be tooweak to be usable. As illustrated in FIGS. 6B and 6C, the voltage pickup651 extends vertically partially into the annular dielectric cavity 661above and below the slit region 671 of the cavity 661. The cavity isformed by the groove of the metal top cover 663 above and the respectivegroove in the metal bottom cover 666 below. While the groundedconductive inner walls of these grooves interpose between the conductivering of the voltage pickup 651 and the central axis of the central hole621 for the inner conductor, the grounded metal does not completelyshield the voltage pickup 651. There is a dielectric slit 671 separatingthe top cover 663 from the bottom cover 666, illustrated in FIGS. 6B and6C. The slit 671 is seen as the dielectric region shaped like acylindrical disk in the mirror plane M (indicated by a dashed line inFIG. 6C) because there is no inner conductor present in the central hole621. When the inner conductor would be in place, the dielectric slit 671would be shaped like an annular disk around the inner conductor. Theunshielded radial electric field in the dielectric slit 671 would besensed by capacitive coupling between the inner conductor and thecentral portion of the ring-shaped voltage pickup 651. The voltagepickup 651 may now provide a usable electrical signal proportional tothe RF voltage of the inner conductor at that location.

The function of the current pickup coil 647 is to sense the RF currentin the inner conductor at the center of the central hole by sensing acirculating magnetic field threading through the coil in a directionparallel to the circular central axis of the toroidal mandrel 642. ByFaraday's Law, an oscillating electrical signal is induced in the coil,proportional to the oscillating magnetic flux in the toroidal mandrel642 enclosed within the turns of the conductive wire of coil 647.According to Ampere's Law, the strength of the magnetic field threadingthrough the current sensor 641 is proportional to the total currentcrossing the area of the plane enclosed within the central hole of thetoroidal current sensor (analogous to the doughnut hole of a doughnut).As is true for any coaxial structure, the current through the innerconductor at any location of an RF pipe is exactly equal to an oppositecurrent in the outer conductor. The sensor casing 665 of the V-I sensor600 may be considered to be the equivalent outer conductor of the RFpipe, where the inner conductor is passing through the central hole 621.Accordingly, the current sensor 641 may not function properly unless theRF current in the sensor casing 665 is constrained to flow outside ofthe circular disk shaped region enclosed by the outer circumference ofthe toroidal mandrel 642. For example, if the top cover 663 and thebottom cover 666 come in electrical contact at a radial distance fromthe central axis that is shorter than the inner radius of the toroidalmandrel 642 then a fraction of the current in the conductive casing mayflow through that contact. This current being opposite to the current inthe inner conductor, would diminish the magnitude of the total currentenclosed by the current sensor 641, hence diminish the magnetic fluxthreading through the coil 647. If the total current through the contactinside the area enclosed by the toroidal current sensor 641 is too lowthen the magnetic field may be insufficient to induce a usableelectrical signal in the current pickup coil 647. Again, the dielectricslit region 671 prevents electrical contact between the top cover 663and the bottom cover 666 at radial distances smaller than the innerradius of the toroidal current sensor 641, as illustrated in FIGS. 6Band 6C.

An uninterrupted, continuous dielectric region separating the top cover663 and the bottom cover 666 all the way to the outer circle of thetoroidal current sensor 641 is achieved by designing the voltage pickup651 to be smaller than the vertical height of the cavity 661. Theconductive ring of the voltage pickup 651 may be positioned roughlysymmetrically between the top cover 663 and the bottom cover 666 bysupporting parts comprising insulating materials. Accordingly, in alldirections, the immediate vicinity of the conductive voltage pickup 651is insulating material. As described above, this insulating material iswithin the slit region 671 of the cavity 661. The shape of thedielectric above the conductive ring of the voltage pickup 651 isdelineated by a zig-zag dashed line FIG. 6B. It may be noted that, asillustrated by the dashed lines in FIG. 6C, the zig-zag dielectric slitregion 671 is present both above and below the voltage pickup 651because the conductive ring of the voltage pickup 651 has to beelectrically isolated from the grounded sensor casing 665.

The current pickups of the current sensors are generally shielded fromthe RF electric field by grounded conductive parts. Shielding thecurrent pickup is advantageous in applications where the electric fieldis strong and the magnetic field is weak, such as close to a highimpedance load. In V-I sensor 600, the current pickup coil 647 islocated in the dielectric cavity 661 inside the sensor casing 665. Theconductive parts encountered while moving radially inward from thetoroidal current sensor 641 to the inner conductor include first, theconductive voltage pickup 651 and second, a portion of the inner wall ofthe conductive sensor casing 665, as seen in FIGS. 6B and 6C and alsodescribed above. These interposing conductive parts may help shield thecurrent sensor 641 from the radial electric field. Some of the electricfield lines emanating from the inner conductor may terminate on thegrounded inner wall of the conductive sensor casing 665. In addition,the conductive voltage pickup 651 serves a dual purpose by partiallyshielding the coil 647 from the RF electric field. Since the voltagepickup 651 is not shorted to ground, the reduction in the electric fieldprovided by the conductive ring depends on the magnitude of theimpedance to ground at the center terminal 654.

The structures of both the current sensor 641 and voltage pickup 651 ofV-I sensor 600 are axisymmetric relative to a shared axis passingthrough the center and in a direction normal to the plane of the centralhole 621. Furthermore, both the current sensor 641 and voltage pickup651 share the same mirror plane (indicated by a dashed line M in FIG.6C) perpendicular to the longitudinal axis. The symmetry of thestructure of V-I sensor 600 helps reduce/eliminate any discrepancy inthe measurement of the phase angle (Φ) between voltage and current.Furthermore, first order cancellation effects due to axisymmetry makethe sensor output signals of V-I sensor 600 less sensitive to machiningtolerances and to positioning errors during assembly.

FIG. 6D illustrates an example toroidal structure that may be used asthe mandrel 642 of the toroidal current sensor 641 shown in FIG. 6B. Thetoroidal mandrel 642 has a continuous groove on its outer surface inwhich a conductive wire may be placed to form the coil 647 (shown inFIG. 6B). The two opposite ends of the coil 647 may be threaded throughtwo openings 643, illustrated in FIG. 6D, and subsequently attached tothe pair of coaxial connectors 645 (see FIG. 6B). The voltage pickup 651(see FIG. 6B) may be connected by a conductive element threaded througha hole in the toroidal mandrel 642 and the opening 653 to be attached tothe center terminal 654.

The toroidal mandrel 642 comprises plastic and other insulatingmaterials and may be fabricated using, for example, 3D printingtechnology. After the coil 647 has been mounted on the grooved toroidalmandrel 642, the structure may optionally be encased in a coating ofresin using, for example, an embedded-resin technique. The resinencapsulation firmly fixes the coiled multi-turn current pickup 647.

The integrated assembly of the V-I sensor 600, described above,comprising the current sensor 641 and the combined electric-field shieldand a voltage pickup 651 provides the advantage of a compact V-I sensordesign.

FIGS. 7A through 7E illustrate a current sensor assembly 701, similar indesign to that of V-I sensor 600. Unlike the V-I sensor 600, the currentsensor assembly 701 does not sense voltage. Also, the design of thetoroidal mandrel 742 used for the current sensor assembly 701 isdifferent from the grooved toroidal mandrel 642, as described furtherbelow.

FIG. 7A illustrates a perspective view of a current sensor assembly 701using a toroidal current sensor 741 placed in a dielectric cavitybetween a top cover 782 and a bottom cover 784 of the sensor casing 765.The current sensor 741 is described further below with reference toFIGS. 7C-7E. The top and bottom covers 782 and 784 may comprise a metal(e.g., copper or aluminum). The current sensor assembly 701 has acentral hole 710. The inner conductor of a coaxial transmission line(e.g., a RF pipe) for which the current sensor assembly 701 may be usedwould be passing through the central hole 710. The current sensorassembly 701 would be thereby positioned symmetrically about alongitudinal axis of the coaxial transmission line.

FIG. 7B illustrates an exploded view of the current sensor assembly 701.In FIG. 7B, the current sensor 741 has been removed from of the sensorcasing 765 to show the structure of the bottom half of the dielectriccavity 720 and the bottom cover 784. (The top half of the structure isdescribed further below with reference to FIG. 7C.) The dielectriccavity 720 may be partitioned into an outer dielectric region 723 and aninner dielectric region, referred to as the zig-zag dielectric slit 725.The outer dielectric region is the region above the outermost groove inthe floor of the bottom cover 784. Outside the outer circle of thisoutermost groove, the metal top cover 782 and the metal bottom cover 784may be connected together physically and electrically, but no electricalcontact may be made between the top cover 782 and the bottom cover 784inside the outer circle of the outermost groove.

The zig-zag dielectric slit 725 comprises the dielectric region over thetwo grooves on either side of a conductive ridge 750 shaped like a ringprotruding from the floor of the bottom cover 784. The conductive floorof the dielectric cavity 720, including the conductive ridge 750 wouldbe electrically and physically separated from the respective conductiveroof of the dielectric cavity 720 by an unbroken continuous dielectricregion. Accordingly, the top of the conductive ridge 750 may protrudeinto a respective groove in the top metal cover 782 but may not makecontact with the roof. The combined top and bottom portions of thezig-zag dielectric slit 725 would thus be a zig-zag shaped dielectricregion going around and over the conductive ridge 750, as indicated by azig-zag dashed line in FIG. 7B.

FIG. 7C illustrates a portion of an RF system 700 comprising an innerconductor 711 of an RF pipe passing through the central hole 710 of thecurrent sensor assembly 701 positioned symmetrically around the innerconductor 711. The grounded outer conductor would be attached physicallyand electrically to the top cover 782 from above and the bottom cover784 from below, thereby grounding the sensor casing 765. The sensorcasing would act as the grounded outer conductor for the portion of theinner conductor 711 passing through the central hole 710, similar to thesensor casing 665 of the V-I sensor 600.

In FIG. 7C, the current sensor assembly 701 is illustrated by a cutawaydiagram of an exploded view that includes the current sensor 741. Thecurrent sensor 741 comprises the toroidal mandrel 742, along with theconductive current pickup coil 747. The toroidal mandrel 742 comprises asolid dielectric material with a winding passage. The winding passagemay be accessed through access holes 749 at various locations on thesurface of the toroidal mandrel 742. An attachment 743 having two holeshas been placed over one of the access holes 749. The two opposite endsof the conducting wire of the coil 747 are shown protruding upwardthrough the holes in the attachment 743. A portion of the toroidalmandrel 742 has been cut out to show the conducting wire of the coil 747tunneling through the winding passage in the solid dielectric materialof the toroidal mandrel 742. The coil 747 is inlaid inside the mandrel742. The design of mandrel 742 provides greater mechanical supportrelative to the grooved design of mandrel 642, thereby eliminating theresin encapsulation step described earlier with reference to FIG. 6D.

In FIG. 7C, the toroidal mandrel 742 has been placed in the outerdielectric region 723 and the respective groove on the floor of thebottom cover 784 (see FIG. 7B). The exploded view of the current sensorassembly 701 illustrates that the top half of the toroidal mandrel 742may fit into a groove in the top cover 782 in the outer dielectricregion 723 of the dielectric cavity 720. The conductive ridge 750 maylikewise extend into an adjacent groove in the top cover 782 in thezig-zag dielectric slit 725 of the dielectric cavity 720. The conductiveridge 750 being a continuous ring interposed between the toroidalcurrent sensor 741 and inner conductor 711 may effectively shield thecurrent sensor 741 from the RF electric field.

As explained above, electrical contact between the grounded top cover782 and the grounded bottom cover 784 in the region encircled by thetoroidal current sensor 741 would diminish the strength of the magneticfield threading through the current pickup coil 747 and may excessivelyweaken the output signal of the current signal. So, the top of theconductive ridge 750 is electrically isolated from the top cover 782 bythe zig-zag dielectric slit 725. The zig-zag shape of the dielectricregion is indicated by a zig-zag dashed line in FIG. 7C.

The cutaway diagram of a portion of an RF system 700, illustrated inFIG. 7D, shows the current sensor assembly 701 with the top cover 782fitted over the bottom cover 784. The zig-zag shape of the dielectricslit 725 has been indicated by a dashed zig-zag line in FIG. 7D.

FIG. 7E illustrates a planar view of a bottom portion of the currentsensor assembly 701 and an inner conductor 711 of an RF pipe passingthrough the central hole 710 of the current sensor assembly. The currentsensor 741 comprising the toroidal mandrel 742 and the current pickupcoil 747 is shown over the bottom cover 784. The two opposite ends ofthe coil 747 are passing through holes in the attachment 743. Theattachment 743 may be placed over an opening similar to the access hole749. The ring-shaped conductive ridge 750 is seen interposed between theinner conductor 711 and the current sensor 741. The dielectric slit 725is seen on either side of the conductive ridge 750.

The use of a mandrel, such as mandrels 642 and 742, permits a currentsensor design to use a coil with many turns as the current pickup. Thelarger number of turns increases the sensitivity of the respectivecurrent sensor. The increased sensitivity allows each turn to have asmaller cross section and, thus, the size of the entire current sensormay be reduced, allowing the current sensor to be placed in otherwiseinaccessible areas.

Although the mandrels described in this disclosure are shaped like atoroid, it is understood that other shapes may be used, for example, asquare or a regular polygon having any number of sides. Furthermore,pickups of various shapes could be implemented without the use of amandrel.

The various aspects of the embodiments described in this disclosure maybe applied to fabricate V-I sensors using various other manufacturingtechniques. For example, the current pickup can be manufactured inlayers of dielectric and conductive material linked by vias, such as inprinted circuit board (PCB) technology.

The embodiments of toroidal current sensors described above provide theadvantages of axial symmetry of a torus, higher immunity to noise ofmulti-turn current pickups, and ease of use obtained with compactstructures.

The V-I sensors and measurement methods, described in this disclosure,provide embodiments that may enable very high precision measurements atlow manufacturing cost. High precision at low manufacturing cost may beachieved by including design features intended to reduce the sensitivityof the V-I measurements to machining errors and assembly errors. Theprecision of the current sensors depend on machining tolerance thatcauses variations in the dimensions that determine the geometry of thecurrent pickup (e.g., the area enclosed by the rectangular half-loop).The precision in measuring current may also be limited by assemblytolerances, for example, the precision with which the current pickup maybe placed, including the radial distance from the longitudinal axis andthe angle between the plane of the half-loop and the longitudinal axis.The precision of the voltage measurement is likewise dependent on themachining tolerance (e.g., the accuracy in the diameter andcircumference of the voltage pickup ring) and assembly tolerance (e.g.,the angle between the plane of the ring and the longitudinal axis). Theinventors have performed detailed computer simulations of thesensitivities of the V-I sensor signals to variations in dimensions andplacements of the current and voltage pickups and found that a highprecision of 1% may be achieved for a standard machine and placementtolerance value of 0.005 inches may be achieved. The computersimulations are done using a calibrated 3D finite element solver forMaxwell's equations over a wide range of RF power, RF frequency, andload impedance used in plasma processing.

Example embodiments of this application are summarized here. Otherembodiments can also be understood from the entirety of thespecification as well as the claims filed herein.

Example 1. A radio frequency (RF) system includes a radio frequency (RF)power source configured to power a load with an RF signal; an RF pipeincluding an inner conductor and an outer conductor connected to groundcoupling the RF power source to the load; and a current sensor alignedto a central axis of the RF pipe carrying the RF signal. The currentsensor is configured to monitor the current of the RF signal, andincludes a conductive half-loop disposed proximate the RF pipe, wherethe conductive half-loop includes a first end and an opposite secondend. The current sensor is configured to output an output signal betweenthe first end and the second end. A sensor casing is disposed around theRF pipe, where the sensor casing includes a conductive materialconnected to the outer conductor of the RF pipe. A gallery is disposedwithin the sensor casing and outside the outer conductor of the RF pipe,where the current sensor is disposed in the gallery. A slit in the outerconductor of the RF pipe exposes the current sensor to a magnetic fielddue to the current of the RF signal in the inner conductor of the RFpipe.

Example 2. The system of one of example 1 where the slit has a lengthalong an inner circumference of the outer conductor and a width parallelto the central axis of the RF pipe, and wherein the width is between 0.5mm and 5 mm.

Example 3. The system of one of examples 1 or 2, where, along adirection orthogonal to the central axis of the RF pipe, the conductivehalf-loop includes a first plane of mirror symmetry including thecentral axis of the RF pipe and a second plane of mirror symmetryorthogonal to the first plane of mirror symmetry, and where the firstplane of mirror symmetry of the conductive half-loop and the centralaxis of the RF pipe are co-planar.

Example 4. The system of one of examples 1 to 3, where the conductivehalf-loop includes: a branch aligned parallel to the axis of the RFpipe; a second branch coupled at a first end of the first branch, thesecond branch being orthogonal to the first branch; and a third branchcoupled at a second end of the first branch, the third branch beingorthogonal to the first branch and parallel to the second branch.

Example 5. The system of one of examples 1 to 4, further including:insulating support structures to support the various branches of theconductive half-loop.

Example 6. The system of one of examples 1 to 5, where the RF pipeincludes: an inner conductor electrically coupled to the RF power sourceand the load; and an outer conductor electrically coupled to a referencepotential node.

Example 7. The system of one of examples 1 to 6, further including: afirst voltage sensor to monitor the voltage of the RF signal, thevoltage sensor disposed axisymmetrically around the RF pipe.

Example 8. The system of one of examples 1 to 7, where the first voltagesensor includes: a conductive ring disposed along an inner surface ofthe outer conductor of the RF pipe; and an insulating ring disposedbetween the conductive ring and the RF pipe outer conductor, where theinsulating ring electrically insulates the conductive ring from the RFpipe.

Example 9. The system of one of examples 1 to 8, further including: asecond voltage sensor disposed symmetrically around the RF pipe, wherethe first voltage sensor is located at a first location on the axis ofthe RF pipe, the second voltage sensor is located at a second locationon the axis of the RF pipe, and where a first distance between the firstlocation and a plane of mirror symmetry of the current sensor is aboutthe same as a second distance between the second location and the planeof mirror symmetry.

Example 10. A radio frequency (RF) system including: a radio frequency(RF) power source configured to power a load with an RF signal; an RFpipe including an inner conductor and an outer conductor connected to areference potential node coupling the RF power source to the load; and afirst voltage sensor disposed axisymmetrically around an axis of the RFpipe carrying the RF signal, the first voltage sensor being configuredto monitor the voltage of the RF signal.

Example 11. The system of example 10, further including: a secondvoltage sensor disposed symmetrically around the RF pipe, where thefirst voltage sensor is located at a first location on the axis of theRF pipe, the second voltage sensor is located at a second location onthe axis of the RF pipe.

Example 12. The system of one of examples 10 or 11, further including: acurrent sensor disposed around the RF pipe at a third location, thecurrent sensor being aligned to the axis of the RF pipe carrying the RFsignal, the current sensor being configured to monitor the current ofthe RF signal.

Example 13. The system of one of examples 10 to 12, where the thirdlocation is disposed between the first location and the second location.

Example 14. The system of one of examples 10 to 13, where the currentsensor includes a conductive half-loop including a first end and anopposite second end, where, along a direction orthogonal to the axis ofthe RF pipe, the conductive half-loop includes a first plane of mirrorsymmetry including the axis of the RF pipe and a second plane of mirrorsymmetry orthogonal to the first plane of mirror symmetry, and where thefirst plane of mirror symmetry of the conductive half-loop and the axisof the RF pipe are co-planar.

Example 15. The system of one of examples 10 to 14, where a firstdistance between the first location and the plane of mirror symmetry isabout the same as a second distance between the second location and theplane of mirror symmetry.

Example 16. A method of measuring a radio frequency (RF) signal, themethod including: having a current sensor aligned to an axis of an RFpipe carrying an RF signal, the current sensor being disposed in agallery that is disposed within a sensor casing and outside an outerconductor of the RF pipe, the sensor casing being disposed around the RFpipe, the current sensor including a conductive half-loop, theconductive half-loop including a first end and an opposite second end;and determining a current of the RF signal based on measuring an outputsignal between the first end and the second end.

Example 17. The method of example 16, where the RF pipe includes aninner conductor electrically coupled to an RF power source and a load,and an outer conductor, where the method further includes grounding theouter conductor.

Example 18. The method of one of examples 16 or 17, further includinghaving a first voltage sensor disposed axisymmetrically around the RFpipe; and determining a voltage of the RF signal based on measuring anelectrical signal at a terminal of the first voltage sensor.

Example 19. The method of one of examples 16 to 18, further including:having a second voltage sensor disposed symmetrically around the RFpipe, where the first voltage sensor is located at a first location onthe axis of the RF pipe, the second voltage sensor is located at asecond location on the axis of the RF pipe; and measuring anotherelectrical signal at a terminal of the second voltage sensor, where thevoltage of the RF signal is determined based on the electrical signaland the another electrical signal.

Example 20. The method of one of examples 16 to 19, where the conductivehalf-loop includes a first plane of mirror symmetry including the axisof the RF pipe and a second plane of mirror symmetry orthogonal to thefirst plane of mirror symmetry, and where the first plane of mirrorsymmetry of the conductive half-loop and the axis of the RF pipe areco-planar.

While this invention has been described with reference to illustrativeembodiments, this description is not intended to be construed in alimiting sense. Various modifications and combinations of theillustrative embodiments, as well as other embodiments of the invention,will be apparent to persons skilled in the art upon reference to thedescription. It is therefore intended that the appended claims encompassany such modifications or embodiments.

What is claimed is:
 1. A radio frequency (RF) system comprising: a radiofrequency (RF) power source configured to power a load with an RFsignal; an RF pipe comprising an inner conductor and an outer conductorconnected to ground coupling the RF power source to the load; a currentsensor aligned to a central axis of the RF pipe carrying the RF signal,the current sensor being configured to monitor the current of the RFsignal, the current sensor comprising a conductive half-loop disposedproximate the RF pipe, the conductive half-loop comprising a first endand an opposite second end, the current sensor being configured tooutput an output signal between the first end and the second end; asensor casing disposed around the RF pipe, wherein the sensor casingcomprises a conductive material connected to the outer conductor of theRF pipe; a gallery disposed within the sensor casing and outside theouter conductor of the RF pipe, wherein the current sensor is disposedin the gallery; and a slit in the outer conductor of the RF pipe toexpose the current sensor to a magnetic field due to the current of theRF signal in the inner conductor of the RF pipe.
 2. The system of claim1, wherein the slit has a length along an inner circumference of theouter conductor and a width parallel to the central axis of the RF pipe,and wherein the width is between 0.5 mm and 5 mm.
 3. The system of claim1, wherein, along a direction orthogonal to the central axis of the RFpipe, the conductive half-loop comprises a first plane of mirrorsymmetry comprising the central axis of the RF pipe and a second planeof mirror symmetry orthogonal to the first plane of mirror symmetry, andwherein the first plane of mirror symmetry of the conductive half-loopand the central axis of the RF pipe are co-planar.
 4. The system ofclaim 1, wherein the conductive half-loop comprises: a first branchaligned parallel to the central axis of the RF pipe; a second branchcoupled at a first end of the first branch, the second branch beingorthogonal to the first branch; and a third branch coupled at a secondend of the first branch, the third branch being orthogonal to the firstbranch and parallel to the second branch.
 5. The system of claim 4,further comprising: insulating support structures to support the first,the second, and the third branches of the conductive half-loop.
 6. Thesystem of claim 1, wherein the inner conductor is electrically coupledto the RF power source and the load; and wherein the outer conductor iselectrically coupled to a reference potential node.
 7. The system ofclaim 1, further comprising: a first voltage sensor to monitor thevoltage of the RF signal, the first voltage sensor disposedaxisymmetrically around the RF pipe.
 8. The system of claim 7, whereinthe first voltage sensor comprises: a conductive ring disposed along aninner surface of the outer conductor of the RF pipe; and an insulatingring disposed between the conductive ring and the RF pipe's outerconductor, wherein the insulating ring electrically insulates theconductive ring from the RF pipe.
 9. The system of claim 8, furthercomprising: a second voltage sensor disposed symmetrically around the RFpipe, wherein the first voltage sensor is located at a first location onthe central axis of the RF pipe, the second voltage sensor is located ata second location on the central axis of the RF pipe, and wherein afirst distance between the first location and a plane of mirror symmetryof the current sensor is about the same as a second distance between thesecond location and the plane of mirror symmetry.
 10. A radio frequency(RF) system comprising: a radio frequency (RF) power source configuredto power a load with an RF signal; an RF pipe comprising an innerconductor and an outer conductor connected to a reference potential nodecoupling the RF power source to the load; and a first voltage sensordisposed axisymmetrically around an axis of the RF pipe carrying the RFsignal, the first voltage sensor being configured to monitor the voltageof the RF signal, the first voltage sensor being located at a firstlocation on the axis of the RF pipe; and a current sensor disposedaround the RF pipe at a second location on the axis of the RF pipe, thecurrent sensor being aligned to the axis of the RF pipe carrying the RFsignal, the current sensor being configured to monitor the current ofthe RF signal.
 11. The system of claim 10, further comprising: a secondvoltage sensor disposed symmetrically around the RF pipe, wherein thesecond voltage sensor is located at a third location on the axis of theRF pipe.
 12. The system of claim 11, wherein the second location isdisposed between the first location and the third location.
 13. Thesystem of claim 11, wherein the current sensor comprises a conductivehalf-loop comprising a first end and an opposite second end, wherein,along a direction orthogonal to the axis of the RF pipe, the conductivehalf-loop comprises a first plane of mirror symmetry comprising the axisof the RF pipe and a second plane of mirror symmetry orthogonal to thefirst plane of mirror symmetry, and wherein the first plane of mirrorsymmetry of the conductive half-loop and the axis of the RF pipe areco-planar.
 14. The system of claim 13, wherein a first distance betweenthe first location and the second plane of mirror symmetry is about thesame as a second distance between the third location and the secondplane of mirror symmetry.
 15. A method of measuring a radio frequency(RF) signal, the method comprising: having a current sensor aligned toan axis of an RF pipe carrying an RF signal, the current sensor beingdisposed in a gallery that is disposed within a sensor casing andoutside an outer conductor of the RF pipe, the sensor casing beingdisposed around the RF pipe, the current sensor comprising a conductivehalf-loop, the conductive half-loop comprising a first end and anopposite second end; and determining a current of the RF signal based onmeasuring an output signal between the first end and the second end. 16.The method of claim 15, wherein the RF pipe comprises an inner conductorelectrically coupled to an RF power source and a load, and an outerconductor, wherein the method further comprises grounding the outerconductor.
 17. The method of claim 15, further comprising having a firstvoltage sensor disposed axisymmetrically around the RF pipe; anddetermining a voltage of the RF signal based on measuring an electricalsignal at a terminal of the first voltage sensor.
 18. The method ofclaim 17, further comprising: having a second voltage sensor disposedsymmetrically around the RF pipe, wherein the first voltage sensor islocated at a first location on the axis of the RF pipe, the secondvoltage sensor is located at a second location on the axis of the RFpipe; and measuring another electrical signal at a terminal of thesecond voltage sensor, wherein the voltage of the RF signal isdetermined based on the electrical signal and the another electricalsignal.
 19. The method of claim 18, wherein the conductive half-loopcomprises a first plane of mirror symmetry comprising the axis of the RFpipe and a second plane of mirror symmetry orthogonal to the first planeof mirror symmetry, and wherein the first plane of mirror symmetry ofthe conductive half-loop and the axis of the RF pipe are co-planar.